1. Field of the Invention
This invention relates to electronic assemblies surrounded by enclosures that prevent passage of electromagnetic or radio frequency interference (RFI). Enclosing structures combine shielding covers with ground planes associated with the electronic assemblies. Shielding covers derive from shaped composite sheets comprising nonwoven mats of randomly oriented, low melting, metal fibers. The invention further relates to means for attaching shielding covers to ground planes to provide completed shielding enclosures.
2. Description of the Related Art
Electronic assemblies, containing interference-sensitive or signal generating devices, require isolation by shielding covers and ground planes to protect the devices or prevent damage by the signals they emit. Shielding covers often include an electrically conducting element as part of a composite. A variety of composites containing both metal and polymeric materials are known for use in many varied applications. Composites may include metal in the form of continuous sheet, perforated sheets, mesh, woven screen or non-woven webs of randomly distributed fibers. Similarly, polymer structures, combined with the various forms of metal, may include films, sheets, perforated sheets, woven material or non-woven layers with random fiber distribution. Regardless of the metal/polymer composite used, it must act as a shield for electromagnetic and radio frequency waves. The interference caused by such waves in electronic devices is commonly referred to as electromagnetic interference (EMI) or radio frequency interference (RFI), hereinafter jointly referred to as EMI.
Effective EMI shielding requires the formation of a uniform conductive enclosure around the EMI-sensitive or EMI-emitting device. U.S. Pat. No. 5,294,826 (Marcantonio et al) discloses a combined heat dissipation and laminated shielding cover which absorbs energy by magnetic effects and electro-conductive effects to shield against electromagnetic interference. The laminated cover combines layers of metallic copper on either side of a magnetic layer.
Suppression of radiated emissions from individual integrated circuits may involve the use of a shielded housing over an apparatus, e.g. a circuit board, that carries multiple integrated circuits. Such a housing is described in U.S. Pat. No. 4,661,888 (Jewell et al). The housing of U.S. Pat. No. 4,661,888 and the cover of U.S. Pat. No. 5,294,826 require attachment to a ground plane using conventional methods such as soldering to a grounded contact point or connection through a conductive gasket or multiple machine screws. These methods include time-consuming, additional steps.
An EMI shielding layer, associated with the conductive enclosure, may be in the form of a continuous layer or a discontinuous grid, such as a metal mesh or nonwoven fibrous metal mass. A continuous layer, such as a metallic plate, is the most effective for EMI shielding because no gaps exist to allow passage of EMI. However, when using a discontinuous grid, any enclosure formation process that significantly increases the maximum void dimension in the shielding layer, sometimes called the "slot effect", could cause faulty EMI shielding performance of the shielding material. Void size increases in a variety of ways including, e.g. when the grid is stretched or by damage from tearing or other processes that can break the grid structure.
Previous disclosures reveal ways of producing and shaping sheet material to provide covers that have EMI shielding capability, typically using electrically conducting layers, which are required in many applications.
For example U.S. Pat. No. 3,272,292, (Nicely), discloses a non-woven unitary metallic sheet which is fabricated by extruding a molten stream from a metallic melt into an atmosphere which reacts to form a stabilizing film about the periphery of the metal stream. The spun metal filaments are allowed to solidify, and then collected as a nonwoven fibrous mass. The mass of filaments is then compressed into a sheet-like form, and given strength by binding all or selected adjacent fibers together.
U.S. Pat. No. 4,689,089 (Gaughan) discloses an EMI shielding sheet comprising a layer of nonwoven reinforcing fibers which supports a layer of metal whiskers or fibers formed from a ductile metal or metal alloy. The EMI shielding sheet is suitable for shaping of covers by stamping. Another stampable EMI shielding construction appears in U.S. Pat. No. 4,678,699 (Kritchevsky et al). This patent notes that, "The shielding layer must be able to maintain its shielding effectiveness upon stamping." Kritchevsky further states that, "Holes formed upon tearing can dramatically reduce shielding." This statement refers to tearing of the shielding layer. Such statements reflect the fact that stamping processes tend to disrupt fibrous networks, breaking the fibers which, in the case of EMI shielding, results in poorer shielding effectiveness of the metal layers. There is no teaching of how to reduce fiber or filament breakage to a minimum during shaping.
Stamping is one method for forming shaped EMI shielding structures. This forming technology was developed in the metal industry for forming thin metal objects. It involves rapid, almost instantaneous application of mechanical force to distort a sheet into a shaped object. Stampable plastic/metal composite sheets may require heating, to soften the plastic surrounding the metal shielding layer, prior to stamping. This reduces the modulus of the plastic, allowing it to flow while the metal shielding composite responds to the high pressure, shaping force of the stamping press. The speed of this process demands high levels of ductility for the metal and high plasticity for the remainder of the composite, to absorb the applied force without rupture. This method, applied to sheet molding compound (SMC), provides automotive body panels and business machine housings using reinforced material comprising a non-woven, glass-fiber reinforcing layer, and a mat containing conductive fibers for EMI shielding, held together with a resin such as polyester. The SMC is a flat sheet prior to forming in compression dies of high tonnage presses. Material properties limit the use of SMC to simple, relatively shallow shapes. Conditions used for sharp draws, e.g. multiple rib formation in the shaped panel, may cause ripping of the shielding layer and reduction of EMI shielding performance.
As a substitute for stamping, the use of thermoforming or injection molding may be considered. Thermoforming, as it relates to the present invention, comprises heating a sheet and forming it into a desired shape. The process includes heating a thermoplastic composite sheet until it becomes soft and pliable, then using either air pressure or vacuum to deflect the softened sheet towards the surface of a mold until the sheet adopts the shape of the mold surface. Upon cooling, the sheet sets in the required shape allowing removal from the mold. Disclosures in JP 1990-276297 (Nakanishi) suggest the use of vacuum formable EMI shielding sheets, employing sandwich structures of brass filaments between plastic films. One embodiment uses a non-woven cloth of synthetic resin to reinforce the brass filaments. Reinforcement involves needle-punching of the metal fibers into the non-woven web. Application of plastic film, on both sides of the reinforced shielding layer, completes the vacuum formable sheet. Information from JP 1990-276297 includes no evidence of the condition of the EMI shielding layer after vacuum forming.
European patent EP 529801, commonly assigned with the instant application, discloses EMI shielding, add-on sheets, comprising carrier material with a metal fiber mat at least partially embedded in the carrier material. The add-on sheets provide EMI shielding covers to selected parts of a thermoformed structure. Successful use of these add-on sheets requires that they possess or develop porosity when thermoformed in contact with the thermoformable substrate blank to which they were applied. This reference addresses changes that can occur during thermoforming to impair the effectiveness of the EMI shield. These changes include breakage or beading of molten metal fibers and in extreme cases separation of the metal matrix into discontinuous islands which provide no EMI shielding. While observing a progression of damaging conditions there is no evidence of means for optimum stabilization of the EMI shield during thermoforming.
Depending on the melting point of the metal fibers, it is possible for fibers in an EMI shielding layer to melt, and rupture under the stress of stretching and shaping during thermoforming. If sufficient molten fibers break in close proximity to each other the liquid metal may flow to form a metal droplet by coalescence of multiple molten fibers. At some point in metal droplet development its size is sufficient to bleed through the surface of the EMI shielding layer. Continued flow of molten metal into a droplet produces a defect extending outside the plane of the shielding layer. Upon cooling the defect is clearly seen as a shiny protruding bead, bump or disc extending from, and disposed over the surface of the thermoformed structure. Formation of these spheroidal protrusions can adversely affect EMI shielding because the flow of molten metal, resulting in their formation, draws metal from an area surrounding the protrusion causing reduced shielding efficiency in that area. In addition, current conducting, solid spheroidal protrusions cause potential electrical shorting problems if they contact circuit elements or microdevices in the restricted space usually associated with housings for electronic assemblies. Solid protrusions can also interfere with injected resin flow when the shielding composite is an insert for injection molding.
Ideally, a shaped EMI shielding article employs an electrically conductive element, within its structure, to contain generated IEMI or present a barrier to radiated EMI. At the same time the conductive element should not establish electrical contact with an electronic component, thereby causing a device to malfunction. However, effective EMI shielding enclosures require electrical continuity between a shaped EMI shield and a ground plane associated with, e.g. a printed circuit. To maintain electrical insulation of the shield, but allow access for electrical connection to the conductive element, commonly assigned patent EP 529,801 indicates the need for special connectors. Surprisingly, it was discovered that, by carefully controlling the thickness of insulation covering the electrically conducting shield, to less than 0.15 mm, the need for special connectors may be eliminated in favor of a simpler thermal bonding technique, also referred to herein as heat staking.
Several alternative solutions have been attempted to improve the effectiveness of conductive fiber based EMI shielding. The formation of pressure welds or sintered bonds between the fibers improves electrical conductivity, but reduces overall flexibility and extensibility of the welded mat. Composite metal-fiber/polymer sheets containing such sintered metal mats cannot be thermoformed without breaking many of the fibers themselves, the bonds between the fibers or both, thus reducing the shielding properties at higher stretch ratios required in thermoformed parts.
With the increasing use of advanced, EMI-sensitive electronics, a need exists for improved materials for shaping into EMI shielding covers or housings that reliably protect electronic assemblies. Methods to shape EMI shielding covers rely upon the use of moldable composites. Many shapes limit stretching of the moldable composites to shallow drawn structural features. In other cases the shapes require composites with ability to retain shielding capabilities even when complex shapes demand localized elongation of 300-500%. This condition is possible using composite structures of the invention comprising a thermoplastic sheet of carrier material, supporting a layer of randomly distributed, low melting metal fibers stabilized against fiber rupture and development of bumps during forming, by means of a coating (fiber-coating) of a thermoplastic polymer. Composite sheets of the coating (fiber-coating) of a thermoplastic polymer. Composite sheets of the invention provide improved EMI shielding performance by maintaining the integrity of molten metal strands during thermal shaping to introduce shallow to deep drawn features into the composite sheet.
Prior art clearly reveals problems, associated with loss of EMI shielding, when the shielding layer loses its integrity through, e.g. tearing, excessive hole formation, fiber separation and increase in the number and size of gaps. While claiming EMI shielding effectiveness, in excess of 30 dB, for a variety of formable sheets, prior art advances no teaching of properties tending to stabilize such sheets against the problems outlined above. Also there is no evidence of the variability of shielding performance of formed prior art structures. Accordingly, there is a need not only to provide effective EMI shielding but to control it in such a way that formed structures consistently show almost no evidence of undesirable features, such as tears, holes, breaks, and spheroidal protrusions such as beads, bumps and the like.
The current inventors discovered how to stabilize randomly oriented metallic fiber mats to maintain fiber continuity and conductivity to consistently provide thermoformed covers with maximum shielding capability. This contributes dual benefits of improved EMI shielding performance coupled with the potential for cost savings from reduced incidence of rejection of shielded structures after thermoforming. Additional cost savings and convenience accrues from the use of a heat staking method to connect shielding covers to conductive ground planes.